Use empty initializer lists for zeroing structs.

This works around an incorrect Clang warning:
https://llvm.org/bugs/show_bug.cgi?id=21689

Bug 15387371

Change-Id: I2460a4f6dd414f518789be1fcc8ce8b205c6a066
Reviewed-on: https://swiftshader-review.googlesource.com/4491
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>

src/Main/FrameBuffer.cpp

diff --git a/src/Main/FrameBuffer.cpp b/src/Main/FrameBuffer.cpp
index 9afd19f..bb1151f 100644
--- a/src/Main/FrameBuffer.cpp
+++ b/src/Main/FrameBuffer.cpp
@@ -193,8 +193,7 @@
 
 	void FrameBuffer::copyLocked()
 	{
-		BlitState update = {0};
-
+		BlitState update = {};
 		update.width = width;
 		update.height = height;
 		update.destFormat = destFormat;